This invention relates to bearings used in combination rotary-vibratory pile drivers or drills, in particular sonic drills or pile drivers, and to a method for preloading such bearings.
Rotary-vibratory drills employ a vibratory force superimposed upon a rotary action to accomplish the drilling operation. Such drills are mainly advantageous for increasing drilling speed when drilling overburden and rock in geological drilling operations, for example during placer exploration.
Sonic drills are rotary-vibratory drills where the vibration frequency is in the sonic range, typically between 50 and 120 Hertz. The frequency range is chosen to achieve high drilling rates and also to allow the vibrations to coincide with the resonant frequency of the drill string or steel pile. If the machine does not operate at resonance, as more and more weight of drill pipe is added, the amplitude of the tip of the drill bit is reduced to such a point that little power is transmitted and drilling does not proceed further.
Until the advent of sonic drills, the only methods available to sample gold placer properties have been cable tool percussion drilling (also known churn drilling or Keystone drilling), continuous sample recovery with air rotary and surface bulk sampling. All of these methods suffer disadvantages which have been addressed by the development of sonic drills. However there are other applications for sonic drilling such as installation of concrete piles, water well drilling, rock drilling for blast holes and rock coring.
However, problems have been encountered in finding a bearing assembly for rotary-vibratory drills in general, and sonic drills in particular, which is capable of transmitting the vibratory forces encountered, while accommodating the rotary motion.
One type of bearing assembly used in the past employs a stem-like inner member surrounded by an annular outer member. Angular contact ball bearings are fitted between the two members. A vibratory device is placed on either the outer member or the inner member while the other member rotates. The drill string is connected to the rotating member. It is essential to stop play developing in the bearings during drilling operations. The inner races of the sets of bearings are typically held between a shoulder at one end the inner member and a nut at the other end of the inner member. The outer races of the bearings are held apart by an intervening portion of the outer member. Preloading is accomplished by forcing the bearings towards each other and tightening the nut so as to tension the inner member and likewise compress the portion of the outer member between the outer races. This preloading places a considerable force across the bearings even before vibratory forces are encountered. Thus the total force on the bearings is the initial preloading plus the oscillating vibratory force. The maximum forces encountered are relatively high and lead to premature failure of the bearings.